1. Field of the Invention
The present invention relates to a semiconductor device in which electrodes and wiring lines made only or mainly of aluminum are formed, as well as to a manufacturing method of such a semiconductor device.
2. Description of the Related Art
In recent years, with an increased demand for active matrix liquid crystal display devices, techniques for forming a thin-film transistor (hereinafter abbreviated as “TFT”) on a glass substrate, which is inexpensive, have been developed rapidly. In an active matrix liquid crystal display device, a TFT (hereinafter referred to as “pixel TFT”) provided for each of millions of pixels that arranged in matrix controls charge entering and exiting from the associated pixel electrode by its switching function.
An integrated circuit is now common in which TFTs (called “circuit TFTs” for convenience) for driving the pixel TFTs are incorporated in peripheral driver circuits and a display pixel section including the pixel TFTs and a driver circuit section including the circuit TFTs are formed on the same substrate.
This type of integrated circuit includes millions of pixel TFTs and more than several hundred circuit TFTs. It is natural that an integrated circuit of such a configuration have a problem of a low production yield. For example, if a single pixel TFT does not operate properly, the pixel electrode connected thereto loses its function as a display element. This causes what is called a point defect. In the case of a normally-black liquid crystal display device, a point defect causes a black point in a white display area, which much deteriorates a visual impression.
On the other hand, if a circuit TFT does not operate properly, all the pixel TFTs that receive a drive voltage from that circuit TFT do not function as switching elements. This causes what is called a line defect, which is fatal to the liquid crystal display device.
Therefore, in an active matrix liquid crystal display device, millions of TFTs are required to continue their normal and stable operations over a long time. However, it is currently very difficult to completely eliminate point detects and line defects.
The above-mentioned point defect and line defect are mainly caused by an operation failure of a TFT. One of the main causes of TFT operation failures is a contact failure. The contact failure occurs at an electrical connecting portion (hereinafter called a contact) between a wiring electrode and a TFT active layer (composed of thin-film semiconductor layer) or a gate electrode. In particular, the contact failure is serious in planar TFTs in which electrical connection between a wiring electrode and a TFT is taken through a narrow hole (contact hole).
The contact failure is the main cause of early degradation in the semiconductor device characteristics. The degradation is particularly accelerated in a case of large-current or high-temperature operation. This is the reason for an extreme notion that the reliability of contacts determines that of a semiconductor device.
In general, in the pixel display region of an active matrix liquid crystal display device, there exist only contacts to TFT active layers because gate electrodes themselves extend outside the pixel display region.
On the other hand, the peripheral driver circuits include hundreds of thousands to millions of contacts. The contacts in the peripheral driver circuits are required to be more reliable than those in the pixel display region, particularly because gate electrodes have contacts and a large-current operation causes temperature increase.
Causes of contact failures are generally classified into the following three categories.
The first cause is such that a conductive film constituting a wiring electrode and a semiconductor film constituting the source or drain of a TFT do not form an ohmic contact. For example, this is caused by an insulative coating such as a metal oxide film formed on the junction surface. Further, the states (impurity concentration, density of defect energy states, cleanliness, etc.) of the surface and its vicinity of a semiconductor film greatly influence the contact performance.
The second cause is such that poor coverage of a conductive film constituting a wiring line causes a disconnection in a contact hole. This type of contact failure needs to be remedied by employing a proper method and conditions for forming a wiring electrode.
The third cause is a disconnection of a wiring electrode resulting from the sectional shape, for instance, of a contact hole, which strongly depends on the conditions for etching an insulating layer (SiN, SiO2, etc.) covering the contact portion. In particular, a side recess and blowholes that are formed by overetching cause serious problems because they very much deteriorate the coverage. To illustrate one of those problems, a description will be made of how a side recess is formed in a gate electrode with reference to FIGS. 1A-1C.
FIGS. 1A-1C are enlarged views of a contact hole for taking contact between a gate electrode of a planar thin-film transistor and a wiring line.
In FIG. 1A, reference numeral 101 denotes a member made of a metal material capable of being anodized, more specifically, a gate electrode made of a material mainly made of Al (aluminum). For simplicity, a gate insulating film, a semiconductor layer, etc. existing under the gate electrode 101 are not shown in FIGS. 1A-1C.
Reference numeral 102 denotes an anodic oxide film (mainly made of Al2O3) formed by anodizing the gate electrode 101 in an electrolyte. Being very dense and strong, the anodic oxide film 102 serves to protect the gate electrode 101 from heat applied thereon in a heat treatment, to thereby suppress occurrence of hillocks and whiskers. Hillocks and whiskers are needle or prickle-like protrusions formed by abnormal growth of aluminum.
An interlayer insulating film 103, which may be a silicon oxide film, a silicon nitride film, a silicon oxynitride film, or the like, is formed on the gate electrode 101.
As shown in FIG. 1A, a contact hole 104 is formed through the interlayer insulating film 103 by etching it by wet etching or dry etching.
To complete the contact hole 104, it is necessary to etch the interlayer insulating film 103 which may be a silicon oxide film, a silicon nitride film, a silicon oxynitride film, or the like, and then etch the anodic oxide film 102.
However, since the anodic oxide film 102 is very dense and strong, it takes certain time to etch it. Therefore, in the case of isotropic etching, the etching proceeds also laterally to a considerable extent, to form a side recess 105 as shown in FIG. 1B.
Formation of a wiring electrode 106 in this state results in a structure shown in FIG. 1C. Since the side recess 105 cannot be covered with the wiring electrode 106 completely, there is a possibility of disconnection. Further, this often causes a contact failure.
If the overetching at the end of the etching of the anodic oxide film 102 is too long, the gate electrode 101 is etched little by little, possibly forming blowholes. This may also cause a contact failure.
Further, upon exposure of the aluminum gate electrode 101, a natural oxide film is formed on its surface. The existence of the natural oxide film may also cause a contact failure.
Although the above problems can be avoided by using, as the electrode material, a metal material other than aluminum, a silicide material, or some other proper material, such a solution is not always proper in view of the low-resistivity characteristic of aluminum.